Changing a material effects both text objects
Most likely this is because both text objects are assigned the same material (this could happen if you duplicated one from the other).  If a material (or any other datablock, i.e. particle system, image, etc.) has multiple users it will display the number of users in a small box to the right of the material name.  To make a single user copy of the material simply click on that box.


The material "Material" has 2 users, clicking on the 2 box will create a new material with all the same properties and name it "Material.000".  Changing this new material will not affect the original material "Material".